The present invention relates generally to the fields of chemical vapor deposition (CVD) using low pressure techniques and the growth of diamond films. More particularly, it concerns the use of microwave plasma-assisted CVD (MPCVD) to grow epitaxial diamond films at high temperatures (&gt;1600.degree. C.).
The growth of homoepitaxial diamond films for integrated and optoelectronic applications is currently an active area of diamond research. (Ravi, 1993). Metastable synthesis of high quality single-crystal diamond from the vapor phase is complicated by several technological and theoretical factors. Among these are the incorporation of large concentrations of optically active defect centers (Collins and Lawson, 1989; Clark et al., 1992) (many involving nitrogen), and the lack of an accurate model for nucleation, growth, and surface chemistry during diamond film deposition.
The primary goal for large-scale production of diamond-based electronics is to develop the most reliable and economical deposition technology possible. Chemical vapor deposition (CVD) using low pressure techniques, such as microwave plasma-assisted CVD (MPCVD), are attractive for their reasonable capital investment, process flexibility and automation, and their demonstrated ability to produce high quality epitaxial films. (Sato and Kamo, 1992). In addition, given a sufficient growth rate (.gtoreq.20 .mu.m/hr), diamond homoepitaxy could be used in the enlargement, modification, or repair of existing natural and high pressure/high temperature synthesized (HPHT) crystals for use in many applications, such as repair of diamond anvils for high pressure research. (Vohra and Vagarali, 1992).
The most common characterization techniques for determining the phase purity and crystallinity of diamond films is by Raman spectroscopy. (Bachmann and Wechert, 1991; Knight and White, 1989). The absolute position and full width at half-maximum (FWHM) of the first order zone-center phonon mode (found at .about.1332.5 cm.sup.-1 in natural type IIa diamond) have been shown to be a semiquantitative means of evaluating the degree and range of crystalline order, and residual stress present in diamond films. (Knight and White, 1989; Boppart et al., 1985). More difficult, but fully quantitative methods of determining the crystallinity and crystallographic orientation of a film or crystal are Laue, x-ray, or electron diffraction techniques. (Bachmann and Wechert, 1991). Defect and impurity complexes are usually studied by photoluminescence (PL) or cathodoluminescence (CL), and are labeled by the characteristic energy E.sub.zpl (Energy of the "zero phonon line") of the zero phonon (purely electronic) transition of the center. (Clark et al., 1992).
The most common defect/impurity complexes found in both bulk and thin film diamond involve nitrogen (substitutional or interstitial) and lattice vacancies. Recent studies using isotope labeled precursor gases have indicated the enhanced incorporation of nitrogen-related defects at the film/substrate interface, as determined by intensity maxima in the ZPLs of two defect centers at 2.16 eV (575 nm) and 2.21 eV (560 nm). (Behr et al., 1993).
It has been generally understood that the growth of epitaxial diamond could not be accomplished with low pressure CVD and combustion processes at substrate temperatures in excess of 1200.degree. C. (Bachmann and Lydtin, 1991). This belief, based in part on thermal desorption studies of diamond powders, was that the upper temperature limit would be due to the desorption of atomic hydrogen and the subsequent reconstruction and graphitization of the diamond surface. (Matsumoto et al., 1981; Pate, 1986). This belief was further reinforced by observations of graphitic inclusions and/or growth on diamond and non-diamond substrates held at temperatures above 1000.degree.-1100.degree. C. in low pressure CVD environments. (Spitsyn et al., 1981; Zhu et al., 1989).
The highest substrate temperature for the epitaxial growth of diamond was reported by Snail and Hanssen (1991). They reported diamond growth with a substrate temperature range of 1150.degree.-1500.degree. C. on natural diamond seed crystals using a laminar, premixed oxygen-acetylene flame in air. Growth rates of 100-200 .mu.m/hr were observed for lower temperatures. However, the seed crystals subjected to higher deposition temperatures had larger misorientations and the diamond growth at 1500.degree. C. exhibited a low growth rate and strong graphitic character in the center of one of the seed crystal's faces. Additionally, this reference discusses an open atmosphere method of producing diamond films which leads to contamination of the diamond film by graphite and nitrogen and is therefore disfavored.